Variable valve actuation (VVA) systems are used to actively control the timing and lift of engine valves to achieve improvements in engine performance, fuel economy, emissions, and other characteristics. Depending on the means of the control or the actuator, VVA systems are classified as mechanical, electrohydraulic, and electromechanical (sometimes called electromagnetic). Depending on the extent of the control, they are classified as variable valve-lift and timing, variable valve-timing, and variable valve-lift. They are also classified as cam-based or indirect acting and camless or direct acting. In the case of a cam-based system, the traditional engine cam system is kept and modified somewhat to indirectly adjust valve timing and/or lift. In a camless system, the traditional engine cam system is completely replaced with electrohydraulic or electromechanical actuators that directly drive individual engine valves. All current production automotive variable valve systems are cam-based, although camless systems will offer broader controllability, such as individual valve control and cylinder or valve deactivation, and thus better fuel economy.
The most prevailing design of an electromechanical VVA (or EMVVA) actuator includes an armature moving longitudinally between first and second electromagnets, a rod connected with the armature and an engine valve, and a pair of actuation springs attached to the rod and urging or centering the moving mass to a zero spring force or neutral position when the armature is not latched on either of the electromagnets. The engine valve is kept to closed and open positions when the armature is latched to the first and second electromagnets, respectively. For a simple, full-lift valve actuation, this spring-mass pendulum system is energy efficient, with the springs storing and releasing potential energy and the moving mass accumulating and releasing kinetic energy.
The prevailing EMVVA design does have several problems or potential problems. One of them is its power-off state. When engine power is off, the net spring force of the two actuation springs keeps the engine valve half open and the armature at the middle point between the two electromagnets. In certain vehicle regulations, it is required to keep engine valves closed at power-off. Also, to initialize an EMVVA actuator at the start of power-on, great effort and a large amount electrical current are spent to pull the armature from the middle point to either of the two electromagnets because of the nonlinear nature of the electromagnetic force. Therefore, it is desirable to keep the engine valve at the closed position and the armature near the first electromagnet.
With its fixed placement of the electromagnets and the actuation springs and nonlinear magnetic forces, prevailing EMVVA actuators also have trouble actuating an engine valve with a short stroke or lift, which is generally desirable and in some cases necessary for low load and idle engine operations. Some prevailing EMVVA actuators may perform short-lift actuation, but at great expense of electrical energy sustaining a large electromagnetic force through a substantial air gap to counter the spring centering force. This additional electrical energy further stretches the limit of a vehicle electrical system, especially during low load and idle operations when the vehicle alternator or electrical generator is the least efficient.
Disclosed in U.S. Pat. No. 5,996,539, assigned to FEV Motorentechnik GmbH & Co KG, is an EMVVA actuator including an adjusting device to vary the valve strokes. The adjusting device supports and controls the displacement of a base of the opener spring, thus controlling the pre-stress of the two actuation springs and the neutral position of the armature. At the least and most pre-stressed states of the actuation springs, the engine valve operates at partial and normal strokes, respectively. The design has the potential to resolve the valve stroke variability issue associated with most EMVVA designs. However, it fails to provide a solution to meet the need to keep the engine valve closed at power-off , and it also entails an additional hydraulically-operated-and-controlled locking mechanism, which incurs added complexity and reliability concern, to stabilize the adjusting device for partial stoke operations.